Air cushion vehicles are generally of two basic types, one type having a flexible or partially flexible skirt to contain lifting air, which leaks out under the skirt and can be controlled to some extent for balance and steering. Usually the propulsion and directional control are obtained by separate means, such as propellers and rudders, on top of the vehicle. The other basic type has rigid side walls surrounding an air chamber, the walls being immersed in or in sliding contact with the supporting surface to minimize air leakage. The supporting air can be used for propulsion, since the energy is well contained, but additional propulsion means is often used.
A primary problem with all air cushion vehicles is stabilization and directional control, since there is no appreciable contact with the supporting surface. The air cushion provides a very low friction support and the vehicle is easily displaced by small disturbances. Small air cushion vehicles, in particular, are affected by movement of the occupants and can be steered, although unreliably, by offsetting the load or balance. For effective control it is necessary to have complete control over the pressure distribution and direction of exit of the air flow from the air cushion supporting the vehicle.